The present invention relates to a process for the fabrication of a phase mask for using ultraviolet laser light to make diffraction gratings in optical fibers used for optical communications, etc., a phase mask, an optical fiber with a Bragg grating, and a dispersion compensating device.
In optical fiber communication systems, enormous amounts of information must be transmitted at very high speeds. An optical fiber system comprises an optical signal source for conveying information, an optical fiber transmission line and a receiver for demodulating information that light signals convey.
For optical communication systems, optical fibers made of high-purity silica are generally used as transmission media. Typically, a conventional optical communication system is designed to transmit light signals using a certain wavelength range. In that system, a component of longer wavelength is subjected to a longer delay in propagation time than that of shorter wavelength. So far, however, this dispersion has not caused considerable deterioration of light signal information. This is because a single channel selected from a wavelength region with reduced dispersion has been used in early-stage systems.
As encountered in wavelength division multiplexing (WDM) systems representative of high-speed transmission technologies using optical fibers, a number of channels must now be used over a wider wavelength region so as to convey a great amount of information. With this, it is required to make more precise compensation for group velocity dispersion. For instance, when it comes down to a WDM system, compensation for dispersion in association with an increased number of channels provided therein becomes an increasingly important challenge.
Recently, for a Bragg diffraction grating formed in an optical fiber there has been demanded a chirped grating in which the grating pitch increases or decreases linearly or nonlinearly depending on the position of the direction of right-angle intersection (or repetition) of grating. Such gratings, for instance, are used as high-reflecting mirrors with a widened reflecting zone, delay time control means, etc.
To address the increasing demands for such chirped gratings, the inventor has come up with a process for the fabrication of an optical fiber-processing phase mask by providing a grating form of repetitive groove-and-strip pattern which is provided on one surface of a transparent substrate, and irradiating an optical fiber with light diffracted by the repetitive groove-and-strip pattern to make a diffraction grating in the optical fiber by interference fringes obtained by interference of diffracted light of different orders of diffraction, wherein a plurality of patterns comprising grooves and strips at varying pitches are written in a row, thereby making said grating form of repetitive groove-and-strip pattern (see JP-A 11-72631).
With a grating prepared using the phase mask fabricated by this process, however, there is a problem that noises are produced in reflection spectra by reason of stitching errors in a writing device used for mask fabrication, and with a grating for dispersion compensation, a problem is that ripples in group delay characteristics, crosstalks, etc. take place, producing an adverse influence on device performance.